Influence Of Tube Voltage On Radiation Dose CT Attenuation And CT Perfusion

Objective:The purpose was to assess the influence of tube voltage and tube current on CT attenuation measurement, radiation dose, and image quality using serial solutions with different iodine concentrations. Materials and Methods:Infuse 4 ml contrast medium (Ultravist,370mgl/ml, Bayer) for each time into 1000 ml normal sodium, and draw 5ml solutions with a syringe after misce bene. Twenty-five syringes with different solutions were fixed on the CT calibration water phantom with the diameter of 20 cm. Fifteen protocols with different tube voltages (120kV,100kV, and 80kV) and tube currents (250mA,200mA,150mA,100mA, and 50mA) were scanned. The other scan parameters included axial scan mode and 1 s/rot for gantry rotation. No other technique for radiation dose adjustment was adopted. The CT data were transferred to Advantage Workstation 4.4 for GE. Region of interest (ROI) with 10mm2 were drown to measure the CT attenuations of serial solutions under different scan settings. Images with standard setting of 120kV and 250mA were measured first to select the serial solutions, CT attenuations of which were from 0HU to 450HU with increment of 5HU.113 solutions were brought into the study, and CT attenuation under other settings were measured and recorded. The changes of tube voltage and tube current influence on CT attenuation were analyzed, and the relationships were calculated. CT dose indexs (CTDIvol, mGy) were recorded and dose length product (DLP, mGy·cm) were calculated. Results:CT attenuations with fixed tube voltage and different tube current were not different (120kV, F=0.001, p=1.000;100kV, F=0.008,p=1.000;80kV, F=0.075,p=0.990). However, CT attenuations with fixed tube current and different voltage tube were different (250mA, H=17.906,p=0.000;200mA, H=17.688,p=0.000;150mA, H=13.527,p=0.000; 100mA, H=20.124,p=0.000; 50mA, H=23.5635,p=0.000).The relationship of CT attenuations for one solution with different tube voltages was calculated and equations were obtained. The relationship between radiation dose and image noise was also analyzed. The key point was obtained to prove that low tube voltage and high tube current can reduce radiation dose. Conclusions:Tube current had no significant effect on CT attenuation measurement, while tube voltage had effect on CT attenuation measurement. Low tube voltage and high tube current could reduce radiation dose while the image quality would be assuranced.Objective:The purpose was to verified the relationship between tube voltage and CT attenuation obtained in partⅠ, and assess the feasibility of CT perfusion with low tube voltage. Materials and Methods:There were 34 patients accepted CT perfusion examination for liver diseases in our deapartment, and 12 patients were excluded for body weight>75kg.22 patients were included in our study, and 7 were female,at age 32-86 years,at weight 43-75kg (averaged 63±11kg). All the patients were divided into two groups:low tube voltage group(80kV) and standard tube voltage group (120kV). There were 11 patients in each group. GE Lightspeed VCT XT 64-slice spiral CT was used. Low tube voltage group used 80kV and 500-650mA, standard tube voltage used 120kV and 200-280mA. The other parameters included axial scan mode, gantry rotation 0.4s/ro, slice thickness 5.0mm, matrix 512×512, FOV 30cm-35cm.17 scans were performed and 16 images were obtained for each time. The time interval was 2.8s and the total time for scan was 50s. Contrast medium was injected into ulnar vein with velocity of 4ml/s and volume of 50ml. Scan was started 10-12s after injection. The CT data were transferred to Advantage Workstation 4.4 for GE and tool of CT perfusion 4 was used for postprocession. ROI with 10mm2 were drown to measure the CT attenuations of aorta, portal vein, and normal liver for the two groups. According to the relationship of equation 2, the calibrated CT attenuation was calculated for low tube voltage group. CT attenuations of the three sets for two groups were transferred to personal computer to calculate and compare the perfusion parameters with special perfusion tool of GE. The perfusion parameters included blood flow (BF), blood volume (BV), capillary permeability surface area product (PS), and hepatic arterial perfusion (HAP). DLPs and effective radiations were calculated and compared. Results:To compare with standard group, perfusion parameters measured with low tube voltage were different:BV(t=-2.295,P=0.028), BF(t=-2.784,P=0.009), PS (t=-3.439,P= 0.002), HAF(t=-2.115,P=0.048). However, the perfusion parameters calibrated were not different:BV(t=-2.015, P=0.068), BF(t=-1.996, P=0.055), PS(t=-3.039, P=0.132), HAF(t=-1.139, P=0.052). The effective dose of two groups was different (t=-4.21,p=0.001). The effective dose of low tube voltage could be reduced by 33%. Conclusions:The perfusion parameters of different tube voltage were different. After calibration by equation 2, the parameters were according to standard result. Moreover, the radiation dose was reduced significantly.